Marinate fish steaks with half turmeric powder and salt for 10 minutes.
Heat mustard oil in a pan and light-fry fish for 1 minute on each side; remove.
Temper nigella seeds and slit green chilies in the same oil.
Add remaining turmeric, water, and salt; bring to a boil.
Add fish pieces, cover, and simmer gently for 6 minutes before serving warm.